National Drought Summary for February 24, 2015

Summary

The upper-level circulation pattern during this U.S. Drought Monitor (USDM) week was dominated by a strong trough over the eastern CONUS (contiguous United States) which funneled cold air masses into the central and eastern United States. Storm systems moving along the southern edge of the trough generated a wintry mix of weather, dumping locally heavy rain and snow from the Lower to Mid-Mississippi Valley on the western end to the Mid-Atlantic coast in the east, improving drought conditions … especially in the Mississippi and Ohio Valleys. The precipitation mostly missed the immediate Gulf of Mexico coastal areas, where drought expanded. An upper-level low pressure system moved across the ridge over the western CONUS and settled in over the Southwest near the end of the week, with above-normal precipitation falling across parts of the Southwest and Rocky Mountains. The week was drier than normal in the Pacific Northwest and much of the Intermountain Basin, where drought expanded. Temperatures in the West averaged above normal this week, but the anomalies were not as warm as in previous weeks.

Hawaii, Alaska, and Puerto Rico

In Hawaii, some stations along the windward slopes of the southern islands received above-normal rainfall, but the week was drier than normal at most other locations. D1 added to the lower slopes of Kauai in response to continued below-normal precipitation and FSA reports of degrading conditions for ranchers. Eastern portions of Puerto Rico received an inch or more of rainfall this week, but areas to the west generally received less than half an inch. The D0 in north central Puerto Rico was expanded a bit to encompass low streams that were just outside the D0 boundary. Three inches or more of precipitation was reported at Alaskan stations along the southern coast and panhandle, but precipitation amounts were generally half an inch or less at most interior and southwestern stations. With temperatures continuing well above normal, no change was made to the depiction in Alaska this week. was expanded a bit to encompass low streams that were just outside the D0 boundary.

The Northeast and Mid-Atlantic

Much of the Northeast and Mid-Atlantic remained buried under a foot or more of snow and shivered with weekly average temperatures 10 to 20 degrees F below normal. Rivers remained frozen, lowering streamflow measurements in some areas to very low levels. A few coastal stations reported precipitation amounts of an inch or more for this USDM week, but precipitation was generally below normal in the Northeast. Above-normal precipitation fell across the Mid-Atlantic States, from West Virginia to Delaware and southern New Jersey, improving streamflow levels. D0 was trimmed in southern West Virginia and adjacent Virginia, but the map depiction remained unchanged across the rest of the region.

The Northern Plains and Midwest

Most of the Central to Northern Plains and Upper Midwest were drier than normal this week, with some areas receiving no precipitation. Precipitation deficits in the Northern Plains to Upper Mississippi Valley continued to mount, exceeding 4 inches over the past 6 months in parts of the Dakotas and Minnesota. But winter is the dry season, and deficits during this time of year are less significant than during the warm season. The drier-than-normal autumn depleted soil moisture and set the state for potential spring concerns, and the current depiction of D0-D1 adequately represents this situation. With temperatures this week continuing well below normal and the ground remaining frozen, no change to the depiction was made in this area.

To the south, a major winter storm moved across the Tennessee and Ohio Valleys, dumping an inch or more of precipitation, mostly in the form of snow, across a wide area, with locally 3 inches or more in parts of southern Kentucky. The precipitation raised stream levels, replenished soil moisture, and erased precipitation deficits for the last 30 days. As a result, D0-D1 were pulled back in Kentucky and the Missouri bootheel. But significant deficits remained at the 60-90 day time scale, so areas of D0 and D1 remained to represent these longer-term conditions. Low stream levels along and north of the Ohio River remained a concern, but no expansion of D0 was made here due to the area receiving above-normal precipitation this week.

The Southern Plains to Southeast

A major winter storm system moved across the Lower to Mid-Mississippi Valley and Southeast during the middle of this USDM week, followed by another system at the end of the week which moved out of the Southern Plains and across the Southeast. These systems left a total of 1-3 inches of precipitation across an area stretching from northeast Texas to the Carolinas, with locally 3-5 inches in parts of Tennessee and northern Mississippi, replenishing soil moisture and filling streams. D0-D2 contracted in Tennessee, northern Mississippi, and adjacent Arkansas. A remnant of D1 remained in western Tennessee, and D1-D2 remained in northeast Arkansas, to reflect the continued significant precipitation deficits which remained over the last 3 to 5 months. The precipitation was enough to prevent further deterioration in northern Alabama to the Carolinas, but not enough to warrant improvement in the drought depiction. The storm systems mostly missed the Gulf of Mexico coastal areas, which generally received less than half an inch of moisture. D1 expanded from southern Louisiana to southern Alabama and the Florida panhandle where weekly precipitation was below normal and deficits continued to mount. Much of this area has very low streamflow levels and precipitation deficits exceeding 6 inches, and locally 9 inches, since the beginning of October.

In Texas, above-normal precipitation at the 7-day to 6-month time scales prompted the pullback of D2-D3 in the northwest corner of the panhandle, while D2-D3 expanded in south-central Texas near the Edwards Plateau. Deeper soil moisture, stock ponds, rivers, and reservoirs in western Oklahoma have never fully recovered from the drought which began 4 years ago. The Washita River has been completely dry for nearly the last 4 years, which local residents say is highly unusual, with the river generally not being without water for more than 90 days in the past. NASA satellite-based measurements of groundwater indicated severely dry conditions in western Oklahoma. D3 was expanded in western Oklahoma across Roger Mills, Custer, Beckham, and Washita Counties to better reflect long-term moisture deficits (over the last 15 to 72 months) as well as record low streamflow levels. D1-D3 were expanded in north central Oklahoma, and D1 in adjacent south central Kansas, where 7-day to 6-month precipitation deficits were greatest and surface water supplies continued to be a concern.

The West

It was another dry week for much of the West, with parts of the Pacific Northwest, Northern California, Arizona, New Mexico, and the Intermountain Basin receiving no precipitation. Scattered areas of the Pacific Northwest and Northern Rockies received half an inch to an inch of precipitation. A slow-moving upper-level weather system brought half an inch to 2 inches of precipitation to parts of Southern California and the Southwest, with 2-4 inches being reported over southwest Colorado. In California, D4 was pulled back in eastern Kern County and adjacent southeast Tulare County, but otherwise The Golden State remained locked in a years-long drought. At the University of California-Berkeley’s Central Sierra Snow Laboratory site near Donner Summit, there was only about a foot of snow on the ground, which is lower for late February than all of the dry winters in the last 70 years. Snow depth at this site never got above 3 feet this winter, when their usual maximum depth would be around 12 feet.

Mountain snowpack remained well below normal, not just in California but all across the Cascades, Sierra Nevada, and Intermountain Basin. The SNOTEL network snow water content ranked among the driest 5 percentile in the historical record for much of this area, with many SNOTEL sites unusually snow free this early in the season. The low mountain snowpack will significantly affect spring and summer water supplies, as melting of the mountain snowpack provides an important water source during the warm season. Mountain snow water content was above normal in only a few parts of the Northern and Central Rockies. D1 expanded along the southern Cascades of Washington, and was added to the Olympic Mountains, to reflect the low snowpack. D1-D3 expanded in southwest Idaho (Owyhee and Canyon Counties) to reflect basin impacts. D0 expanded in southeast Idaho and into southwest Montana and northwest Wyoming (in the vicinity of Yellowstone National Park) where streamflow levels, snow water content, and water-year-to-date precipitation were low. Mountain snow water content and water-year-to-date precipitation were well below normal in northwest Utah to southwest Wyoming. Even though up to an inch of precipitation was observed this week in southwest Utah, NASA satellite-based measurements of groundwater indicated very dry conditions from southwest Utah to southwest Wyoming. As a result, D1 expanded across southwest Utah, D0-D1 expanded in northeast Utah into southwest Wyoming, D2 expanded in northwest Utah, and a spot of D3 was added to northwest Utah.

Looking Ahead

The upper-level circulation pattern (of ridge west/trough east) will undergo a change in the next 7 days. Weather systems will undermine the western ridge, allowing colder-than-normal air to spread westward and encompass most of the CONUS. Some precipitation (a tenth of an inch or more) is expected to fall across most of the CONUS during February 26-March 3. An inch or more is forecast for parts of the Pacific Northwest, Southwest, and Southern to Central Rockies, and for most of the country from the eastern Plains to the East Coast. The eastern storm track is predicted to bring 2 or more inches of precipitation to the Mid-Mississippi Valley to Ohio Valley and Southern Appalachians, as well as the Coastal Carolinas. The driest areas are expected to be the Northern Plains, southwest Texas, and parts of California.

The 6-10 day and 8-14 day outlooks keep the area of below-normal temperatures across most of the CONUS, with only the extreme Southeast, as well as Alaska, warmer than normal. The greatest chances for above-normal precipitation during March 2-10 are expected to be over Alaska and the eastern third of the CONUS. Below-normal precipitation is expected over the West and Northern Plains, spreading across the Rockies and into the Midwest later in the period.

Author(s):Richard Heim, NOAA/NCDC

Dryness Categories

D0 ... Abnormally Dry ... used for areas showing dryness but not yet in drought, or for areas recovering from drought.